Propionic acid and vitamin B12 are two compounds involved in a large number of industrial operations.
As main outlets for propionic acid, there may be mentioned, in particular, the food industry, in which it is employed as a fungicide in the form of calcium and sodium propionates, the cellulose-based plastics industry and the perfumery sector.
Vitamin B12, for its part, is an important cofactor in the metabolism of carbohydrates, lipids, amino acids and nucleic acids. Vitamin B12 is, moreover, a therapeutic agent used in chemotherapy.
Generally speaking, vitamin B12 is prepared by fermentation. The two main corresponding genera of microorganisms employed for its preparation at industrial level are Propionibacterium and Pseudomonas. 
It is noted that, in the standard techniques of production of vitamin B12 using microorganisms of the genus Propionibacterium the growth of these latter becomes impaired during the fermentation process, leading to a fall in productivity with respect to vitamin B12. This is the direct consequence of the concomitant formation of propionic acid in the culture medium. The amount of propionic acid increases during the fermentation process, and reaches a certain limit which inhibits the growth of the said microorganisms.
Traditionally, the industrial production of propionic acid is chiefly carried out by petrochemical methods.
Production by fermentation also proves possible, but is not satisfactory from an industrial standpoint. In general, it employs the assimilation of glucose by propionibacteria and leads to the formation of propionic acid but also of not insignificant amounts of acetic acid. Lastly, according to this fermentation process, low yields of propionic acid are obtained on account of the phenomenon already described above in the case of vitamin B12 production, namely an inhibition of the growth of the Propionibacterium bacteria by propionic acid.